Microstructure and Properties of Fine Grain IN718 Alloy Bar … · 2018-07-18 · diameter bar...
Transcript of Microstructure and Properties of Fine Grain IN718 Alloy Bar … · 2018-07-18 · diameter bar...
MICROSTRUCTURE AND PROPERTIES OF FINE GRAIN IN718 ALLOY
BAR PRODUCTS PRODUCED BY CONTINUOUS ROLLING
1Guosheng Chen,
1Qingzeng Wang,
1Fengjun Liu,
1Zixing Wang,
2Jianxin Dong,
2Xishan Xie
1Special Steel Business Unit, Baoshan Iron & Steel Co. Ltd., Shanghai 200940, China
2High Temperature Materials Research Laboratories, University of Science & Technology
Beijing, Beijing 100083, China
Keywords: IN718 alloy; Hot continuous-rolling bar; Microstructure; Property
Abstract
The newly imported high-alloy steel continuous rolling line in Baoshan Iron & Steel Co. Ltd,
which is the first production line in China, was introduced in this paper. The 14 ~ 55mm
diameter bar hot-rolling procedure has been refined after studying the influences of processing
parameters such as micro-structure of continuous rolling billet, rolling start temperature,
continuous rolling deformation ratio, rolling line rate, and control cooling parameters on
microstructure and properties of IN718 bars. In comparison with traditional open-train mill, this
continuous rolling technology provides fine grain bar products with uniform grain structure and
reasonable distribution of δ phase.
Grain size of bar products can be controlled at ASTM 10 in the center and ASTM 11 ~ 11.5 near
the surface by means of this technology. Now this technology is in a stable production condition
and the bar products produced provide excellent mechanical properties.
Introduction
For superalloy bars produced by traditional open-train mill, the microstructure and property
stability is relatively poor and the products quality is unstable, because the rolling parameters are
difficult to control accurately. For IN718 bar products, such problems are very serious because
its microstructure should be tightly controlled. To produce IN718 alloy with optimum properties,
the grain size should be uniform and maintained at about ASTM 10 and the grain boundaries
should be decorated with a certain amount of granular or rod-like δ phase. Theoretically, to
obtain above mentioned structure, in fine-grained billet, the billet should be heated at the δ phase
solves temperature (1005 ~ 1015ºC [3, 4]) and hot deformed at about 950ºC which is a little bit
higher than the dynamic recrystallization temperature, and the finish processing temperature
should be controlled in the temperature range of 900 ~ 950ºC [5], which is corresponding with
the δ phase precipitation temperature. In the process of engineering hot working practices, such a
technical requirement is easy to be satisfied by die forging and quick upsetting. However, for
producing long product, it is usually very difficult to tightly control the exact temperature.
Generally, for IN718 alloy hot rolled bars 30 ~ 80mm diameter produced through a traditional
horizontal roller, under the condition of fully recrystallization and with proper δ phase
precipitation, the grain size can only be refined to ASTM 7~8 [6].Because of the much higher
controllability of continuous rolling technology compared with the traditional open-train mill, the
structure and property of hot rolled bar products are easy to satisfy optimal value of IN718
technical requirement. Therefore, Baoshan Iron & Steel Co. Ltd imported high-alloy steel bar
continuous rolling line from POMINI of Italy in 2007.
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The continuous hot rolling technology, structure and properties of fine grain IN718 alloy bars
products are introduced in this paper, and in comparison with the same size bar products by
open-train mill.
Continuous Rolling Equipment and Experimental Material
The layer of high-alloy steel bar continuous rolling line of Baoshan Iron & Steel Co. Ltd is
shown in Fig.1. This product line includes: continuous heating-furnace(or batch-type furnace) →
reversing rough rolling mill → continuous tunnel furnace → 16 rack CCR(Compact Cassette
Rolling-mill) continuous rolling mill → cold controlling → cold bed.
The sizes of bars rolled by the continuous rolling mill are ranged from 14mm to 85mm diameter,
dimensional accuracy is ±0.25mm (for < 20mm diameter) and ±0.90mm (for > 80mm diameter).
The cross section of the billet can be within 110 ~ 140mm in edge length, the length can be 2500
~ 4000mm. The billet sizes depend on different type and size bar product requirement. Then, the
original billet should be rolled to various geometric sizes and grain structure requirement by
rough rolling to produce intermediate billet. The heating temperature control of the intermediate
billet is carried out by continuous tunnel furnace. The heating temperature and holding time
should be selected according to bar product specification, size and structure requirements.
Finally the intermediate billet is rolled to finished products by a CCR continuous rolling mill. In
production, different rolling line speed can be selected by certain requirements.
The chemical composition range of experimental IN718 alloys is showed in Table 1. There are
104 groups of bars rolled by open-train mill (horizontal) and 52 groups of bars rolled by
continuous rolling mill for statistical confidence. Heat treatment process: 950 ~ 980ºC/1h/AC +
720ºC/8h, furnace cooled at 50ºC/h to 620ºC/8h/AC.
The bar sizes rolled by open-train mill are: 14 ~ 20mm diameter products rolled by the 250mm
rolling mill, 21 ~ 45mm diameter products rolled by the 300mm rolling mill, ≥ 50mm diameter
bar rolled by the 550mm rolling mill.
Microstructure images in this article, unless specifically marked, the up-down direction of the
picture is the rolling direction.
Table 1 Chemical composition range of experimental IN718 alloys (wt%)
C Cr Mo Nb Ta Al Ti Ni B
0.02-
0.03
18.5-
19.5 3.0-3.2 5.1-5.3 ≤0.1
0.45-
0.55
0.96-
1.10
52.5-
53.5
0.003-
0.005
S P Si Mn Cu Co O N Fe
<0.002 <0.015 <0.35 <0.35 <0.30 <1.0 <0.001 <0.008 Bal
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Figure 1. Lay-out of high-alloy steel bar continuous rolling line
Experimental Results
The Influence of Continuous Rolling Processing Parameter
The continuous rolling mill equipment has been put in production since 2007. The influences of
processing parameters (rolled by rough mill) such as microstructure of continuous rolling billet,
rolling start temperature, continuous rolling deformation ratio, rolling line rate and controlled
cooling methods on microstructure and properties of IN718 bars is studied by numerical
simulation and engineering experiments.
In production of large size bars, to obtain uniform fine grain structure by continuous rolling, fine
grain billet should be prepared by rough rolling. If the billet surface temperature drops too much
during the rough rolling process, the original billet grain size coarser than ASTM 4 will be
retained near the surface of intermediate billet. As shown in Figure 2 and 3 for 45mm diameter
bar production, if there are original coarse grains at the surface of intermediate billet during
rough rolling, although the rolling start temperature and line speed are enhanced at continuous
rolling process, the recrystallized grains at the surface can reach about ASTM 10, but the coarse
grains can not be completely recrystallized.
With a proper rough rolling temperature and line speed at continuous rolling process, there is a
critical deformation ratio which can produce the finest grain size. If the deformation ratio is
higher than this critical value, grains will be coarsened because of the enhanced inner
temperature rising effect [7].
If the billet temperature is too high at the rolling process, then grains grow and the amount of δ
phase will be reduced. In some cases there may be no δ phase at grain boundaries in the bar
center. If the rough rolling temperature is too low or the line speed is too slow, this low
temperature at surface or even at mid-radius incompletely recrystallized grains maybe retained
and unexpected precipitation of δ phase. The above mentioned effect will be more obvious when
the bar size is increased.
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Figure 2. Incompletely recrystallized grain
structure in the zone 5mm distance to the
surface of continuous rolled 45mm diameter
bar. The intermediate billet characterizes with
the grain size coarser than ASTM 4 near
surface. (The left-right direction is the rolling
direction)
Figure 3. Incompletely recrystallized grain
structure in the zone 5mm distance to the
surface of continuous rolling Φ45mm diameter
when the rough rolling temperature or line
speed is too low. (The left-right direction is the
rolling direction)
After producing many groups of IN718 alloy rolled 14 ~ 55mm diameter bars, the optimum hot-
rolling parameters have been refined. Consequently, it is now allowable to produce IN718 alloy
bars with various required grain sizes and δ phase distribution.
2.2 Microstructure and Properties of Fine Grain IN718 Alloy Bar Products Produced by
Continuous Rolling
Fine grain IN718 alloy 14 ~ 55mm diameter bars can be produced with grain size ASTM 10 in
the center by continuous rolling under certain procedures. Grain structure and δ phase
precipitation behavior of 14, 25 and 55mm diameter bars are shown in Figure 4 ~ Figure 9. It is
shown that these grains are uniform and fine from the center to surface. Granular or rod-like δ
phase precipitates homogeneously at the grain boundaries. The precipitation of δ particles near
the surface of the bars seems to be slightly more than that in the center.
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(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 4. Grain structure of 14mm diameter bars produced by continuous rolling
(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 5. Grain structure of 25mm diameter bars produced by continuous rolling
(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 6. Grain structure of 55mm diameter bars produced by continuous rolling
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(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 7. δ phase precipitation at different position of 14mm diameter bars produced by
continuous rolling
(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 8. δ phase precipitation at different positions of 25mm diameter bars produced by
continuous rolling
(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 9. δ phase precipitation at different positions of 55mm diameter bars produced by
continuous rolling
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Figure 10. The comparison of center grain size, stress-rupture (650ºC/690MPa) reduction in area
of different bar sizes produced by CCR and open-train mill (Horizontal)
Figure 10 shows the comparison of grain size, stress-rupture plasticity of different size IN718
alloy bars produced by continuous rolling and open-train mill. At the center of the bars produced
by continuous rolling, the grain size ASTM 10 and stress-rupture plasticity is also very good; but
for the bars produced by open-train mill, the grain size is much larger and the stress-rupture
plasticity is reduced with the increasing size of bars. The grains of different position for different
sizes of IN718 alloy bar rolled by two kinds of rolling methods are shown in Table ІІ. It shows
that fine grain structure (ASTM 10 or finer) can be obtained at the bar center, mid-radius and
surface by continuous rolling. The grain size varied only ASTM 1 ~ 1.5 grade from surface to
center. However the grain size variation between surface and center for bars produced by open-
train mill are relatively large.
Table ІІ ASTM grain size of different size bars rolled by two rolling technology
Rolling
Technology
Φ14mm Φ25mm Φ45mm Φ55mm
Cent. R/2 Surf. Cent. R/2 Surf. Cent. R/2 Surf. Cent. R/2 Surf.
CCR 10 10 11 10 10.5 11 10 10.5 11 10 11 11.5
Horizontal 10.5 11 12 9 9.5 11 8 8.5 10 7.5 8.0 9.5
Note: Surf. means “Surface”, which is 1mm distance to the bar surface (before grinding); R/2
means “Mid-radius”; Cent. means “Center”.
It is also shown in Table ІІ and Figure 10 that if the bars are rolled by open-train mill, only for
bars with the diameters smaller than 25mm diameter a grain structure finer than ASTM 9 can be
obtained. With the increasing of bar sizes, it becomes more and more difficult to obtain grain
structure finer than ASTM 9. Practically, when the bar size is larger than 35mm diameter, it is
very difficult to obtain grain structures finer than ASTM 8.5. Moreover, for the bars smaller than
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14mm diameter processed by open-train mill and 20mm diameter bars under better control for
years, although grain structure reached ASTM 10 or finer, due to long rolling time and quick
temperature drop, the precipitation of needle-like and granular δ phase at the surface, mid-radius
and center position was enhanced, which resulted in reduced room-temperature plasticity.
Table Ш shows mechanical properties of IN718 alloy of 14mm, 25mm, 55mm diameter bars
rolled by two kinds of rolling methods. It can be seen that high-temperature properties of bars
14mm diameter rolled by two kinds of rolling methods varied slightly. However, the room-
temperature plasticity of bars rolled by open-train mill is obviously lower than CCR because of
too much δ phase precipitated. For the 25mm diameter bars, high-temperature properties
especially stress rupture plasticity of bars produced by CCR is higher. For 55mm diameter bars
produced by CCR, the stress rupture plasticity almost doubled when compared to bars produced
by the open-train mill. Moreover, the industrial production statistical data show that
microstructure and mechanical properties are stable for bar production by continuous rolling
technology. The stress-rupture notch sensitivity is not evident with CCR. However in the process
of traditional open-train mill rolling, it often results in stress-rupture notch sensitivity.
(a) at center (b) at mid-radius (c) at 1mm distance to surface
Figure 11. δ phase precipitation behavior at different positions of 14mm diameter bars rolled by
open-train mill
Table Ш Mechanical properties of 14, 25 and 55mm diameter bars rolled by different rolling
methods
Bar Dia.
mm
Rolling
Method
Tensile Results Tensile Results Stress Rupture Testing*
20ºC 650ºC 650ºC/690MPa
US
MPa
El
%
AR
%
US
MPa
El
%
AR
%
Duration
h
El
%
AR
%
14 CCR 1425 26 47 1160 25 50 79 22 45
Horizontal 1470 19 38 1170 27 56 82 24 48
25 CCR 1465 23 44 1150 22 49 78 23 46
Horizontal 1460 22 45 1165 20 41 75 18 30
55 CCR 1435 21 42 1155 26 43 76 20 45
Horizontal 1420 22 47 1145 17 30 80 8 15
* Lasting to 25h after the test, every 12h to increase 35MPa.
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This continuous hot rolling line provided higher accuracy in sizes control, high product yield and
also production efficiency.
3 Discussions
For IN718 alloy bars with fine grain structure produced by traditional open-train mill methods,
the heating temperature must be controlled at a lower level. However, it is difficult to be rolled
through one heat because of difficulties in roll feed. Bar rolled with one heat results in an
incompletely recrystallized, cold-deformation microstructure and needle-like δ phase, which will
embrittle the material because the surface (even center and mid-radius) temperature decreases
too quickly during the long time rolling. The technology of low-temperature heating by open-
train mill can not be widely used in production. Actually in the process of industrial rolling by
open-train mill, generally the heating temperature is nearly 1120ºC, which develops coarse grains.
IN718 alloy billet with original coarse grains of ASTM 2, results in a high deformation resistance
and the dynamic recrystallization can not fully completed even at the condition of more than
80% deformation at 950ºC. During the rolling process, if deformation goes too fast, the
temperature in the bar center positions will increase early during the intermediate rolling process
and the material will develop coarse recrystallized grains. Moreover, the increasing of finish
rolling temperature will decrease the amount of δ phase, possibly totally solution the δ phase.
Such effect will be more obvious for bigger size bars. If this operation is too slow, there will be a
large fraction of cold-deformation microstructure and δ phase precipitation, which may result in
surface cracking and hard roll feed because the surface temperature decreases too fast. This
balance is very difficult to manage when producing the uniform fine grain structure and good
surface quality required for completing rolling process. Except that man-power factors are
extensive, technology parameters such as rolling speed, rolling start temperature, process
temperature and finishing temperature are difficult to be reliably controlled.
The present continuous rolling technology provides high-temperature heating in a continuous
furnace, low-temperature heating in tunnel furnace and selection of the billet size which can
obtain a large amount of deformation and high efficiency, and it can also maintain the original
grain size of billet within a small variation before continuous rolling, providing a decrease in hot-
working resistance and refined grain structure. For different size bars, it is easy to control the
process temperature and finishing temperature at different positions of billet in a proper range by
reasonably choosing, and adjusting technological parameters such as amount of deformation for
rough rolling and continuous rolling, heating temperature and holding time in tunnel furnace,
line speed of continuous rolling and cooling method etc. This provides a relatively uniform fine
grain microstructure, proper amount of δ phase and good mechanical properties for different size
bars rolled with CCR technology. After determination of rolling parameters, it can be
automatically processed, so man-power factors are minimized, controllability and stability are
much better than the traditional open-train mill process.
For small size bars in the process of traditional open-train mill rolling, the effect of inner heating
is small, so it is possible to obtain fine grain structure and high-temperature mechanical
properties similar to continuous rolling technology. However in comparison with continuous
rolling technology, the total rolling time is longer, the temperature drops quickly in the final
rolling stage, resulting in δ phase precipitation, which deteriorates the room-temperature
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plasticity. The microstructures in the center and at the edge of the bars are always
inhomogeneous.
4 Conclusions
(1) IN718 alloy 14 ~ 55mm diameter fine grain, bars rolled by high-alloy steel CCR continuous
rolling line which is located at Special Steel Business Unit of Baosteel, can provide an ASTM 10
grain size in the center of bars, and a completely recrystallized grain structure with ASTM
11~11.5 grain size near the surface. The precipitation of δ phase in different positions of the bars
varied slightly, granular or rod-like δ phase can be uniformly distribute at grain boundaries
providing good mechanical properties.
(2) In comparison with IN718 alloy bar products produced by traditional open-train mill, this
continuous rolling technology eliminates the problems such as grain growth with the increasing
bar size and difficulty to control δ phase. The grain size of 30 ~ 55mm diameter CCR bars can be
refined ASTM 1.5 ~ 2.5 grade with uniform grain structure, such that the precipitation of δ phase
consistent through the cross-section. The mechanical properties of CCR material are better that
processed with the traditional open-train mill, and high-temperature plasticity is obviously
enhanced, which becomes more obvious with increasing of bar sizes.
(3) This continuous rolling technology is stable, man-power influence is small, bar product size
accuracy and production yield are higher than bars produced by traditional open-train mill.
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